Patent Document 1 discloses a vehicle wheel disk having a radially inwardly extending protrusion or rib at a portion radially inside a vent window. As the radially inwardly extending protrusion, Patent Document 1 discloses the following (a) and (b):
(a) a protrusion shown in FIG. 1 of Document 1
The protrusion extends radially inwardly from a first portion to a second portion. The first portion is a portion offset axially more inwardly than a vent window surrounding curved portion which is located radially inside the vent window and is curved axially inwardly. The second portion is a portion located more radially inward than a wheel radial region where hub bolt holes are provided. In a cross section of the wheel taken along a plane perpendicular to a wheel axis, the protrusion has a pair of side walls and a crest wall which connects ends of the pair of side walls and extends straight.(b) a protrusion shown in FIG. 4 of Document 1The protrusion extends radially inwardly from a third portion to a fourth portion. The third portion is a vent window surrounding curved portion which is located radially inside the vent window and is curved axially inwardly. The fourth portion is a portion located more radially inward than a wheel radial region where huh bolt holes are provided. In a cross section of the wheel taken along a plane perpendicular to a wheel axis, the protrusion has a pair of side walls and a crest wall which connects ends of the pair of side walls and has a shape of a convex arc having a single radius.